Soldering flux

ABSTRACT

Disclosed is a soldering flux which prevents a release of an active ion contained in an activator when exposed to a high-temperature atmosphere maintaining at a temperature higher than 80° C., for example, an atmosphere of an engine room of an automobile, and thus makes washing unnecessary. This soldering flux comprises, together with an activator, (1) a thermoplastic resin having a softening point not lower than 80° C., and/or (2) an epoxy group-containing compound, a radical-polymerizable unsaturated double bond-containing compound or a blocked isocyanate group-containing compound, or (3) a thermosetting resin composed of a carboxyl group-containing resin and an epoxy resin and/or a thermosetting resin containing carboxy and epoxy groups.

TECHNICAL FILED

The present invention relates to a soldering flux.

BACKGROUND ART

In general, to coat a solder on a base metal, oxides and other stains onthe surface of the metal are removed, and simultaneously, a re-oxidationof the metal surface during the soldering is prevented, to reduce thesurface tension of the solder and improve the wettability of the meltedsolder with the metal surface, and for this purpose, a soldering flux isused.

As the soldering flux, usually a resin flux formed by incorporating anactivator capable of removing an oxide film into a thermoplastic resinhaving a low softening point, such as rosin, is used.

However, where the soldered portion is exposed to a high-temperatureatmosphere, for example, to an atmosphere of 80° C. as in an engine roomof an automobile, if the flux comprising a thermoplastic resin having asoftening point below about 70° C., such as rosin, the thermoplasticresin is melted and an active ion in the activator held in thisthermoplastic resin is released to lower the electrically insulatingproperty and cause a corrosion of the metal surface.

This problem has been resolved by washing and removing the residual fluxafter the soldering, but since flourocarbon or the like is used as thewashing solvent, a problem arises of environmental pollution, andfurthermore, the cost is increased by the use of the washing solvent andthe washing step.

Therefore, a primary object of the present invention is to provide asoldering flux capable of maintaining the electrically insulatingproperty without washing away the residual flux, i.e., without washing,even if used in a high-temperature atmosphere.

DISCLOSURE OF THE INVENTION

The inventors of the present invention carried out an investigation witha view to attaining the above object, and as a result, found that theabove problem can be solved and the above object can be attained if thesoldering flux is constructed as described below.

In accordance with the first aspect of the present invention, there isprovided a soldering flux comprising at least (a) a thermoplastic resinhaving a softening point not lower than 80° C. and (b) an activator.

If the soldering flux of the first aspect of the present invention isused, the thermoplastic resin having a softening point not lower than80° C. is not melted even if exposed to a relatively high temperaturesuch as the temperature of an engine room of an automobile, andtherefore, no active ion is released. Furthermore, even if a part of thethermoplastic resin is replaced with rosin, the thermoplastic resinforms a high temperature-resistant film on the surface of the flux andcovers the residual rosin after the soldering. Accordingly, the activeion released from the activator, which is formed in a high-temperatureatmosphere is enveloped, and a reduction of the electrically insulatingproperty and the occurrence of corrosion can be prevented.

In accordance with the second aspect of the present invention, there isprovided a soldering flux comprising (a) at least one member selectedfrom the group consisting of epoxy group-containing compounds,unsaturated double bond-containing compounds and blocked isocyanategroup-containing compounds and (b) an activator.

If the soldering flux of the second aspect of the present invention isused, even if the residual flux is not removed by washing, the epoxygroup-containing compound, unsaturated double bond-containing compoundor blocked isocyanate group-containing compound causes, under heating atthe soldering step, an addition reaction with a halogen acid, which isthe active ion of the activator contained in the flux residue, with theresult that the activator is deactivated and the active ion in the fluxresidue eliminated, the resistance to reduction of the electricallyinsulating property and the corrosion resistance are not lowered, andthe reliability is improved.

In accordance with the third aspect of the present invention, there isprovided a soldering flux comprising (a) a thermoplastic resin having asoftening point not lower than 80° C., (b) at least one member selectedfrom the group consisting of epoxy group-containing compounds,unsaturated double bond-containing compounds and blocked isocyanategroup-containing compounds, and (c) an activator.

The invention of the third aspect exerts functions of the inventions ofboth the first and second aspects.

In accordance with the fourth aspect of the present invention, there isprovided a soldering flux comprising at least one thermosetting resinselected from the group consisting of a thermosetting resin compound ofa mixture of a carboxyl group-containing resin and an epoxy resin and athermosetting resin having a carboxyl group and an epoxy group, and anactivator.

If the soldering flux of the fourth aspect of the present invention isused, by the reaction between the carboxyl group and epoxy group, thethermosetting resin component forms a tough crosslinked film at thetermination of soldering, and even if the active ion released from theexcessive activator is left in the film on the surface of the flux,since the epoxy group is contained in the film, this epoxy group reactswith the active ion to include the active ion in the film. Accordingly,even if the flux component remains, the reliability of the solderedportion can be improved without a lowering of the electricallyinsulating property or the corrosion resistance.

According to the present invention, even if the soldered portion is usedin a high-temperature atmosphere without washing away the flux aftersoldering, an excellent electrically insulating property can be attainedand the reliability of the soldered portion can be improved, and thus anexcellent soldering flux can be provided according to the presentinvention. Furthermore, if the present invention is adopted, since it isnot necessary to wash away the flux residue after soldering, the costsare reduced by the omission of the washing step, and environmentalpollution by flourocarbon or the like can be prevented.

BEST MODE OF CARRYING OUT THE INVENTION

A thermoplastic resin having a softening point not lower than 80° C.,which shows a durability at a high temperature, must be selected as thethermoplastic resin to be used in the invention of the first aspect.

The softening point is preferably lower than 230° C. This is because, ifthe softening point is higher than 230° C., the flowability of the fluxat the flow soldering becomes poor. An acrylic resin and astyrene/maleic acid resin are preferably used, and to promote theactivating action, preferably a resin having an acid value of at least20 is used. Accordingly, a polymer obtained by polymerizing a monomerhaving a polymerizable unsaturated group, such as (meth)acrylic acids,esters thereof, crotonic acid, itaconic acid, maleic acid (anhydride),esters thereof, styrene, vinyltoluene, (meth)acrylonitrile,(meth)acrylamide, vinyl chloride or vinyl acetate in the presence of aperoxide catalyst or the like by radical polymerization such as bulkpolymerization, solution polymerization, suspension polymerization oremulsion polymerization, is used.

Halogen acid salts of ethylamine, propylamine, diethylamine,triethylamine, ethylenediamine and aniline, and organic carboxylic acidssuch as lactic acid citric acid, stearic acid, adipic acid anddiphenylacetic acid are preferably used as the activator.

In the first aspect of the present invention, a part of thethermoplastic resin can be replaced by rosin and/or a derivativeheretofore used. As the rosin and/or the derivative, there are usedordinary gum, tall and wood rosin, and derivatives such as heat-treatedrosin, polymerized rosin, hydrogenated rosin, formylated rosin, rosinester, rosin-modified maleic acid resin, rosin-modified phenolic resinand rosin-modified alkyd resin. The rosin and/or the derivative is usedas a binder for uniformly coating a metal activator.

Where the flux of the first aspect of the present invention is used inthe liquid state, a solvent can be further added. A polar solventcapable of dissolving the thermoplastic resin, the activator, andfurther, the rosin component therein, to form a solution is preferablyused, and usually, an alcoholic solvent is used. In view of thevolatility and the activator-dissolving power, isopropyl alcohol isespecially preferably used.

In the soldering flux of the first aspect of the present invention, theamount of the thermoplastic resin having a softening point not lowerthan 80° C. is preferably 0.5 to 80% by weight based on the total flux.If the amount of the thermoplastic resin is smaller than 0.5% by weight,the property of forming a film after soldering is reduced and thehigh-temperature durability becomes poor. If the amount of thethermoplastic resin is higher than 80% by weight, the viscosity of theflux per se increases and because of the film thickness of the flux, aproblem arises of a reduction of the soldering property. Preferably theamount of the activator used in the present invention is 0.1 to 20% byweight based on the total flux. If the amount of the activator issmaller than 0.1% by weight, the activating power becomes too low andthe soldering property becomes poor. If the amount of the activator islarger than 20% by weight, the film-forming property of the flux isreduced and the hydrophilic property is increased, and the corrosionresistance and the electrically insulating property become poor. When asolvent is added to the flux of the present invention, for using theflux in the liquid state, the solvent is preferably used in an amount of20 to 90% by weight based on the total flux. If the amount of thesolvent is smaller than 20% by weight, the viscosity of the flux isincreased and the coating property of the flux becomes poor. If theamount of the solvent exceeds 99% by weight, the amounts of theeffective flux components (the thermoplastic resin and the like) arereduced and the soldering property becomes poor.

As the epoxy group-containing compound used in the second aspect of thepresent invention, there can be mentioned phenolic glycidyl compounds ofthe bisphenol, novolak, alkylphenol and resorcinol types, polyhydricalcohol glycidyl compounds, ester glycidyl compounds, alicyclicepoxides, epoxidized polybutadiene, epoxidized glycerides and epoxidizedfatty acids. Moreover, there can be mentioned epoxy compounds modifiedwith the foregoing compounds. Preferably, a compound having at least twoepoxy groups in the molecule and capable of elevating the melting pointof the flux residue by reaction with a carboxylic acid such as rosinwhen rosin or the like is contained is used.

As the radical-polymerizable unsaturated double bond-containing compoundused in the second aspect of the present invention, there can bementioned (meth)acrylic acid, esters thereof, (meth)acrylonitrile,(meth)acrylamide, vinyl acetate, styrene, vinyltoluene, divinylbenzene,vinylnaphthalene, vinylpyrrolidone, maleic acid, esters thereof,(meth)allyl alcohol, esters thereof, coumarone, indene,dicyclopentadiene, polybutadiene, linoleic acid, and resins modifiedwith these compounds. If a large amount of the compound of this typeremains in the original form in the residue after soldering, thedurability is degraded by softening or the like. Accordingly, acompound, at least 80% by weight of which volatilizes and dissipatesunder heating at the soldering step, or a compound, the molecular weightor melting point of which is increased by a thermal change such aspolymerization, is preferably used.

As the blocked isocyanate group-containing compound used in the secondaspect of the present invention, there can be mentioned isocyanatemonomers such as tolylene diisocyanate, diphenylmethane diisocyanate,hexamethylene diisocyanate and isophorone diisocyanate, and reactionproducts of polymers or isocyanate prepolymers such as polyhydricalcohol adducts with active hydrogen-containing compounds such asphenol, cresol, amines, alcohols, lactams and oximes. In view of the lowvolatility and low toxicity, a blocked compound of an isocyanateprepolymer is preferably used.

As the activator used in the second aspect of the present invention,there can be mentioned halogen acid salts of ethylamine, propylamine,diethylamine, triethylamine, ethylenediamine and aniline. The activatoris necessary for cleaning the metal surface.

Rosin resins such as wood rosin, gum rosin, tall rosin,disproportionated rosin, hydrogenated rosin, polymerized rosin andmodified rosin, and thermoplastic and thermosetting synthetic resinssuch as a polyester resin, an acrylic resin, a urethane resin, an epoxyresin and a phenolic resin, which are customarily used, can beincorporated in the soldering flux of the second aspect of the presentinvention. Organic acids such as lactic acid, citric acid, stearic acid,adipic acid and benzoic acid can be used for promoting the activatingaction. Where the flux is used in a liquid state. an alcoholic solventsuch as isopropyl alcohol or butyl carbitol, an ester solvent such asethyl acetate a hydrocarbon solvent such as toluene and a ketone solventsuch as methylethylketone can be used.

In the second aspect of the present invention, the amount of the epoxygroup-containing compound, radical-polymerizable uncaturated doublebond-containing compound or blocked isocyanate group-containing compoundis preferably 1 to 70% by weight based on the total soldering flux. Ifthe amount of the above compound is smaller than 1% by weight, it isimpossible to sufficiently deactivate the active ion in the activator,and a problem of a lowering of the corrosion resistance or of theinsulating property arises. If the amount of the above compound islarger than 70% by weight, the viscosity of the flux per se is increasedand the coating property of the flux becomes poor, and the thickness ofthe flux coating tends to increase, and the soldering property becomespoor. Moreover, there arises a problem of a reduction of thecompatibility with the activator. The amount of the activator ispreferably 0.1 to 30% by weight. If the amount of the activator issmaller than 0.1% by weight, the activating power is too low and thesoldering property becomes poor. If the amount of the activator islarger than 30% by weight, the film-forming property of the flux becomespoor, and simultaneously, the moisture-absorbing property is increased,resulting in a lowering of the insulating property.

In the third aspect of the present invention, a combination of (a) thethermoplastic resin having a softening point not lower than 80° C., usedin the first aspect, and (b) at least one member selected from the groupconsisting of epoxy group-containing compounds, unsaturated doublebond-containing compounds, and blocked isocyanate group-containingcompounds, used in the second aspect, is used. In the soldering fluxused in the third aspect of the present invention, the amount of thethermoplastic resin (a) having a softening point not lower than 80° C.is preferably 0.5 to 80 % by weight based on the entire flux. If theamount of the thermoplastic resin (a) is smaller than 0.5% by weight,the film property after soldering is lowered and the high-temperaturedurability becomes poor. If the amount of the thermoplastic resin (a) islarger than 80% by weight, the viscosity of the flux per se is increasedand the soldering property is lowered because of the increase of theflux thickness. In the third aspect of the present invention, thecompound (b) is preferably used in an amount of at least 1% by weight.If the amount of the compound (b) is smaller than 1% by weight, theeffect of deactivating the activator is poor, and the corrosionresistance and the electrically insulating property cannot be maintainedat required levels. In the third aspect of the present invention, theactivator is preferably used in an amount of 0.1 to 30% by weight basedon the entire flux. If the amount of the activator is smaller than 0.1%by weight, the activating power is too low and the soldering propertybecomes poor. If the amount of the activator is larger than 30% byweight, the film-forming property of the flux becomes poor and thehydrophilic property becomes strong, and the corrosion resistance andinsulating property becomes poor. Where a solvent is added for using theflux of the third aspect of the present invention in a liquid state,preferably the solvent is added in an amount of 20 to 99% by weightbased on the entire flux. If the amount of the solvent is smaller than20% by weight, the viscosity of the flux becomes too high and thecoating property of the flux becomes poor. If the amount of the solventis larger than 99% by weight, the amount of the effective component(thermoplastic resin or the like) of the flux is too small and thesoldering property becomes poor.

In the forth aspect of the present invention, an acrylic resin, apolyester resin, a urethane resin, an epoxy ester resin and a phenolicresin can be used as the carboxyl group-containing resin. In view of theresistance to a reduction of the insulating property, the hardenabilityand the compatibility, an acrylic resin is most preferably used.

Aliphatic and aromatic epoxy resins can be used as the epoxy resin, anda liquid epoxy resin is preferably used because the liquid epoxy resinpromotes the flowability of the flux.

As the resin containing epoxy and carboxyl groups, there can bementioned a resin formed by reacting hydroxyl groups of an epoxy resinwith a polybasic acid anhydride to form a half ester, and an acrylicresin formed by copolymerizing monomers including glycidyl methacrylate,acrylic acid and the like.

An activator known as a usual flux component, for example, a halogenacid salt of ethylamine, propylamine, diethylamine, triethylamine,ethylenediamine or aniline, or an organic acid such as lactic acid,citric acid, stearic acid or adipic acid, is added to theabove-mentioned resin component.

when the flux is used in a liquid state, a solvent, for example, analcoholic solvent such as ethyl alcohol, isopropyl alcohol,ethylcellosolve or butyl carbitol, an ester solvent such as ethylacetate or butyl acetate, a hydrocarbon solvent such as toluene orterpene oil or a ketone solvent such as acetone or methylethylketone canbe added.

In the fourth aspect of the present invention, a part of thethermosetting resin can be replaced by a thermoplastic resin, wherebythe flowability and film-forming property of the flux at the solderingstep can be controlled. In this case, as the thermoplastic resin, theresins to be used in the first aspect of the present invention can beused, and rosin and derivatives thereof such as polymerized rosin,hydrogenated rosin, disproportionated rosin, rosin-modified maleic acidresin, rosin-modified phenolic resin and rosin-modified alkyd resin canbe used. The mixing ratio of the thermoplastic resin to thethermosetting resin is preferably up to about 80% by weight. If themixing ratio of the thermoplastic resin is higher than 80% by weight.The characteristic properties of the thermosetting resin are lost.

In the soldering flux used in the fourth aspect of the presentinvention, the amount of the thermosetting resin is preferably 0.5 to70% by weight based on the entire flux. If the amount of thethermosetting resin is smaller than 0.5% by weight, the active ion inthe activator cannot be sufficiently deactivated and a problem arisesconcerning the corrosion resistance or the electrically insulatingproperty. On the other hand, if the amount of the thermosetting resin islarger than 70% by weight, the viscosity of the flux per se isincreased, the flux thickness is increased, and the coating property ofthe flux becomes poor. The amount of the activator is preferably 0.1 to30% by weight based on the entire flux. If the amount of the activatoris less than 0.1%, the activating power of the flux is too low and thesoldering property becomes poor. If the amount of the activator islarger than 30% by weight, the film-forming property of the flux becomespoor, and therefore, the moisture-absorbing property is increased and aproblem of a reduction of the electrically insulating property arises.

Examples EXAMPLES 1 THROUGH 4

In Example 1, an acrylic resin was used as the thermoplastic resinhaving a softening point not lower than 80° C. More specifically, astyrene-acrylic resin having an acid value of 160 and a softening pointof 130° C. was used in an amount of 11.1% by weight. Furthermore,disproportionated rosin was used in an amount of 7.4% by weight, and asthe activator, there was used 1.2% by weight of adipic acid and 0.3% byweight of aniline hydrobromide. Moreover, 80% by weight of isopropylalcohol was used as the solvent. These components were sufficientlydissolved and diffused to obtain a flux of Example 1, and thenon-tackiness test, the spreading test, the insulation resistance test,the corrosion test, the solderability test, and the coating adhesiontest were carried out. Note, the non-tackiness test, the spreading test,the insulation test, and the corrosion test were carried out accordingto JIS Z-3197. At the solderability test, a paper-phenolic resinsubstrate printed circuit board (90 mm×135 mm) was soldered by a jetsoldering apparatus and defects were checked by a visual observation. Atthe coating adhesion test, an acrylic coating material was coated on thesoldered portion and subjected to the cold-heat cycle test (-30° C.×30minutes -25° C.×30 minutes; 10 cycles), and peeling or cracking waschecked by a visual observation.

The results obtained in Example 1 are shown in Table 1.

In Example 2, 11.1% by weight of an acrylic resin composed mainly ofmethyl methacrylate, which had an acid value of 130 and a softeningpoint of about 120° C. was used as the thermoplastic resin. Othercomponents were the same as those used in Example 1. The components wereuniformly mixed and dissolved as in Example 1 to obtain a flux ofExample 2. The tests were carried out in the same manner as described inExample 1, and the results are shown in Table 1.

In Example 3, 11.1% by weight of an acrylic resin composed mainly ofisobutyl methacrylate, which had an acid value of 150 and a softeningpoint of 110° C., was used as the thermoplastic resin. Other componentswere the same as those used in Example 1. The components were uniformlymixed and dissolved to obtain a flux of Example 3. The flux wassubjected to the same tests as described in Example 1, and the resultsare shown in Table 1.

In Example 4, a styrene-maleic acid resin having an acid value of 150and a softening point of 110° C. was used as the thermoplastic resin.Other components were the same as those used in Example 1. Thecomponents were uniformly mixed and dissolved to form a flux of Example4. The flux was subjected to the same tests as described in Example 1,and the results are shown in Table 1.

COMPARATIVE EXAMPLE 1

A flux of Comparative Example 1 was prepared by uniformly mixing anddissolving 18.5% by weight of disproportionated rosin, 1.2% by weight ofadipic acid as the activator, 0.3% by weight of hydrobromic anilide and80.0% by weight of isopropyl alcohol as the solvent, and the flux wastested in the same manner as described in Example 1. The results areshown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                                                     Comparative                                       Example No.                 Example No.                                       1      2      3      4      1                            __________________________________________________________________________    Composition                                                                           acrylic resin                                                                              11.1   11.1   11.1   --     --                           (% by weight)                                                                         styrene-maleic acid resin                                                                  --     --     --     11.1   --                           Flux    disproportionated rosin                                                                    7.4    7.4    7.4    7.4    18.5                                 adipic acid  1.2    1.2    1.2    1.2    1.2                                  hydrobromic anilide                                                                        0.3    0.3    0.3    0.3    0.3                                  isopropyl alcohol                                                                          80.0   80.0   80.0   80.0   80.0                         Acid Value of Acrylic or Styrene-Maleic                                                            160    130    160    150    --                           Acid Resin                                                                    Softening Point (°C.) of Acrylic or                                                         130    120    110    110    --                           Styrene-Maleic Acid Resin                                                     Main Componnent in Acrylic Resin                                                                   styrene                                                                              methyl isobutyl                                                                             --     --                                                       methacrylate                                                                         methacrylate                               Test    non-tackiness                                                                              slight cloth                                                                         slight cloth                                                                         slight cloth                                                                         slight cloth                                                                         large cloth                  Results              trace  trace  trace  trace  trace                                Spreading ratio (%)                                                                         93     92     94     93     93                                  insulation                                                                          initial stage                                                                        6 × 10.sup. 12                                                                 2 × 10.sup.12                                                                  5 × 10.sup.12                                                                  4 × 10.sup.12                                                                  .sup. 4                                                                       × 10.sup.10                    resistance                                                                          50 hrs 4 × 10.sup.11                                                                  3 × 10.sup.11                                                                  5 × 10.sup.11                                                                  3 × 10.sup.11                                                                  5 × 10.sup.9                   (Ω)                                                                           500 hrs                                                                              4 × 10.sup.11                                                                  2 × 10.sup.11                                                                  5 × 10.sup.11                                                                  2 × 10.sup.11                                                                  3 × 10.sup.7                   corrosion                                                                           50 hrs no corrosion                                                                         no corrosion                                                                         no corrosion                                                                         no corrosion                                                                         pitting                              resistance                                                                          500 hrs                                                                              no corrosion                                                                         no corrosion                                                                         no corrosion                                                                         no corrosion                                                                         corrosion                            solderability                                                                              below 1                                                                              below 1                                                                              below 1                                                                              below 1                                                                              below 1                              (defect ratio, %)                                                             adhesion to coating agent                                                                  no trouble                                                                           no trouble                                                                           no trouble                                                                           no trouble                                                                           cracking                     __________________________________________________________________________

As apparent from the results shown in Table 1, the flux of the firstaspect of the present invention has an excellent resistance to areduction of the electrically insulating property and the corrosionresistance, compared to the conventional flux.

Furthermore, in Example 1 to 4, the rosin type flux component wasextremely compatible with a thermoplastic resin having a high softeningpoint, i.e. a softening point not lower than 80° C., few solderingdefects were caused, the adhesion to the coating agent was good, and acracking of the film did not occur even at a low temperature. Namely, agood flux was obtained.

Accordingly, the flux is very valuable as a soldering flux and thewashing could be omitted, and therefore, the soldering cost waseffectively reduced.

EXAMPLES 6 THROUGH 9

The following composition was adopted for the flux of Example 6.Polymerized rosin was used in an amount of 11.1% by weight based on theentire flux. A bisphenol type epoxy resin having an epoxy equivalent of470 was used in an amount of 7.4% by weight as the epoxygroup-containing compound. Furthermore, 1.2% by weight of adipic acidwas used as the activator, and 0.3% by weight of propylamine, 50.0% byweight of propyl alcohol and 30.0% by weight of toluene were used. Thesecomponents were uniformly mixed and dissolved, and a flux was obtainedin the same manner as described in Example 1, and the insulationresistance, corrosion resistance and solderability of the flux weretested and the chlorine content in the residue was determined.

The insulation resistance and corrosion resistance were tested accordingto JIS Z-3197. At the solderability test, a paper-phenolic substrate (90mm ×135 mm) was soldered by a flow soldering apparatus, and defects werechecked by the visual observation. The chlorine content in the residuewas determined according to JIS Z-3197 after extraction of solublecomponents with methanol. The results are shown in Table 2.

For the soldering flux of Example 7, polymerized rosin was used in anamount of 11.1% by weight based on the entire flux. An acrylic resinhaving an epoxy equivalent of 430 was used in an amount of 7.4% byweight as the epoxy group-containing compound. The activator and solventwere the same as those used in Example 6. The flux of Example 7 wasprepared by uniformly mixing and dissolving these components, and theflux was tested in the same manner as in Example 6. The results areshown in Table 2.

For the soldering flux of Example 8, polymerized rosin was used in anamount of 13.0% by weight based on the entire flux. Crelan UI (blockedisocyanate supplied by Sumitomo-Bayer Urethane) was used in an amount of5.5% by weight as the blocked isocyanate compound. The activator andsolvent were the same as those used in Example 6. The flux of Example 8was obtained by uniformly mixing and dissolving these components, andthe flux was then tested in the same manner as in Example 6. The resultsare shown in Table 2.

For the soldering flux of Example 9, polymerized rosin was used in anamount of 18.0% by weight based on the entire flux and divinylbenzenewas used in an amount of 5.5% by weight as the unsaturated doublebond-containing compound, and the same activator and solvent as used inExample 6 were used. The flux of Example 9 was prepared by uniformlymixing and dissolving these components, and the flux was tested in thesame manner as described in Example 6. The results are shown in Table 2.

COMPARATIVE EXAMPLE 2

Polymerized rosin was used in an amount of 18.5% by weight, and the sameactivator and solvent as used in Example 6 were used in the same amountsas used in Example 6. These components were uniformly mixed anddissolved to obtain a flux, and the flux was tested in the same manneras described in Example 6. The results are shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________                                                     Comparative                                       Example No.                 Example No.                                       6      7      8      9      2                            __________________________________________________________________________    Composition                                                                           polymerized rosin                                                                          11.1   11.1   13.0   13.0   18.5                         (% by weight)                                                                         bisphenol type epoxy                                                                        7.4   --     --     --     --                           of Flux resin having epoxy                                                            equivalent of 470                                                             acrylic resin having                                                                       --      7.4   --     --     --                                   epoxy equivalent of 430                                                       blocked isocyanate                                                                         --     --      5.5   --     --                                   divinylbenzene                                                                             --     --     --      5.5   --                                   adipic acid   1.2    1.2    1.2    1.2    1.2                                 propylamine hydrochloride                                                                   0.3    0.3    0.3    0.3    0.3                                 toluene      30.0   30.0   30.0   30.0   30.0                                 isopropyl alcohol                                                                          50.0   50.0   50.0   50.0   50.0                         Test    insulation                                                                          initial stage                                                                        5 × 10.sup.12                                                                  6 × 10.sup.12                                                                  5 × 10.sup.12                                                                  4 × 10.sup.12                                                                  .sup. 6                                                                       × 10.sup.11            Results resistance                                                                          after 50 hrs                                                                         3 × 10.sup.12                                                                  5 ×  10.sup.12                                                                 3 × 10.sup.12                                                                  4 × 10.sup.12                                                                  5 × 10.sup.9                   (Ω)                                                                           after 500 hrs                                                                        1 × 10.sup.12                                                                  4 × 10.sup.12                                                                  3 × 10.sup.12                                                                  2 × 10.sup.12                                                                  3 × 10.sup.7                   corrosion                                                                           after 50 hrs                                                                         no corrosion                                                                         no corrosion                                                                         no corrosion                                                                         no corrosion                                                                         pitting                              resistance                                                                          after 500 hrs                                                                        no corrosion                                                                         no corrosion                                                                         no corrosion                                                                         no corrosion                                                                         corrosion                            solderability                                                                              below 1                                                                              below 1                                                                              below 1                                                                              below 1                                                                              below 1                              (defect ratio, %)                                                             chlorine                                                                            before  0.53   0.54   0.53   0.54   0.54                                content                                                                             soldering                                                               (%) in                                                                              after    0.1>   0.1>   0.1>   0.1>  0.43                                residue                                                                             soldering                                                       __________________________________________________________________________

From the results shown in Table 2, it is seen that the flux of thesecond aspect of the present invention was an excellent resistance to areduction of the electrically insulating property and the corrosionresistance, compared to the conventional flux.

EXAMPLES 10 THROUGH 14

For the soldering flux of the third aspect of the present invention, inExample 10, an acrylic resin was used as the thermoplastic resin havinga softening point not lower than 80° C. More specifically, astyrene-acrylic resin having an acid value of 130 and a softening pointof 130° C. was used in an amount of 18.5% by weight, and 1.2% by weightof adipic acid and 0.3% by weight of propylamine hydrochloride were usedas the activator. As the monomer having a radical-polymerizableunsaturated group, 2-ethylhexyl methacrylate, which is a kind of themethacrylic acid ester, was used in an amount of 4.8% by weight. Thesecomponents were sufficiently dissolved and diffused in 75.2% by weightof isopropyl alcohol as the solvent to obtain a flux of the third aspectof the present invention, and the obtained flux was subjected to varioustests, for example, for determining the non-tackiness, the spreadingratio, the insulation resistance, the corrosion resistance, and thechlorine content in the film. The non-tackiness test, the spreadingratio test, the insulation resistance test, and the corrosion test wereconducted according to JIS Z-3197, and the chlorine content wasdetermined according to JIS Z-3197, after an extraction of solublecomponents with methanol. At the solderability test, a paper-phenolicsubstrate (90 mm ×135 mm) was soldered by a jet soldering apparatus, anddefects were checked by visual observation.

The results obtained in Example 10 are shown in Table 3.

In Example 11, 11.1 % by weight of a styrene-acrylic resin and 7.4% byweight of disproportionated rosin were used as the thermoplastic resin,and 1.2% by weight of adipic acid and 0.3% by weight of propylaminehydrochloride were used as the activator. As the monomer, 4.8% by weightof 2-ethylhexyl methacrylate was used, and 75.2% by weight of isopropylalcohol was used as the solvent. These components were sufficiently anduniformly dissolved and diffused. The obtained flux was tested in thesame manner as described in Example 10, and the results are shown inTable 3.

In Example 12, a flux was prepared in the same manner as described inExample 11, except that 4.8% by weight of benzyl methacrylate, which isa kind of the methacrylic acid ester, was used as the monomer, and theobtained flux was tested in the same manner as described in Example 10.The results are shown in Table 3.

In Example 13, a flux was prepared in the same manner as described inExample 11 except that 4.8% by weight of ethylene glycol dimethacrylate,which is a kind of the ester of methacrylic acid, was used as themonomer, and the obtained flux was tested in the same manner asdescribed in Example 10. The results are shown in Table 3.

In Example 14, a flux was prepared in the same manner as described inExample 11 except that a styrene-maleic acid resin having an acid valueof 150 and a softening point of 120° C. was used in an amount of 11.1%by weight as the thermoplastic resin, and the flux was tested in thesame manner as described in Example 10. The results are shown in Table3.

COMPARATIVE EXAMPLE 3

A flux was prepared in the same manner as described in Example 11 exceptthat 18.5% by weight of disproportionated rosin was used as thethermoplastic resin and 80% by weight of isopropyl alcohol was used asthe solvent, and the flux was tested in the same manner as described inExample 10. The results are shown in Table 3.

                                      TABLE 3                                     __________________________________________________________________________                                                           Comparative                                Example No.                        Example No.                                10     11     12     13     14     3                      __________________________________________________________________________    Composition                                                                           1) acrylic resin                                                                          18.5   11.1   11.1   11.1   --     --                     (% by weight)                                                                         2) styrene-maleic                                                                         --     --     --     --     11.1   --                     of Flux acid resin                                                                    disproportionated                                                                         --     7.4    7.4    7.4    7.4    18.5                           rosin                                                                         adipic acid 1.2    1.2    1.2    1.2    1.2    1.2                            propylamine 0.3    0.3    0.3    0.3    0.3    0.3                            hydrochloride                                                                 2-ethylhexyl                                                                              4.8    4.8    --     --     4.8    --                             methacrylate                                                                  benzyl methacrylate                                                                       --     --     4.8    --     --     --                             ethylene glycol                                                                           --     --     --     4.8    --     --                             dimethacrylate                                                                isopropyl alcohol                                                                         75.2   75.2   75.2   75.2   75.2   80.0                   Test    non-tackiness                                                                             no cloth                                                                             no cloth                                                                             no cloth                                                                             no cloth                                                                             no cloth                                                                             large cloth            Results             trace  trace  trace  trace  trace  trace                          spreading ratio (%)                                                                       91     94     92     93     93     --                             insulation                                                                         initial stage                                                                        7 × 10.sup.12                                                                  8 × 10.sup.12                                                                  5 × 10.sup.12                                                                  6 × 10.sup.12                                                                  4 × 10.sup.12                                                                  .sup. 5 ×                                                               10.sup.10                      resistance                                                                         50 hrs.                                                                              6 × 10.sup.12                                                                  6 × 10.sup.12                                                                  4 × 10.sup.12                                                                  3 × 10.sup.12                                                                  3 × 10.sup.12                                                                  5                                                                             × 10.sup.8               (Ω)                                                                          500 hrs.                                                                             6 × 10.sup.12                                                                  6 × 12.sup.12                                                                  3 × 10.sup.12                                                                  3 × 10.sup.12                                                                  1 × 10.sup.12                                                                  6                                                                             × 10.sup.7               corrosion                                                                          50 hrs.                                                                              no corrosion                                                                         no corrosion                                                                         no corrosion                                                                         no corrosion                                                                         no corrosion                                                                         pitting                        resistance                                                                         500 hrs.                                                                             no corrosion                                                                         no corrosion                                                                         no corrosion                                                                         no corrosion                                                                         no corrosion                                                                         corrosion                      solderability                                                                             below 1                                                                              below 1                                                                              below 1                                                                              below 1                                                                              below 1                                                                              below 1                        (defect ratio. %)                                                             chlorine                                                                           before  0.54   0.54   0.54   0.53   0.54   0.55                          content                                                                            soldering                                                                (%) in                                                                             after   0.1>   0.1>   0.1>   0.1>   0.1>   0.43                          residue                                                                            soldering                                                        __________________________________________________________________________

As apparent from the results shown in Table 3, according to the thirdaspect of the present invention, a flux having superior variouscharacteristics to those of the conventional flux can be provided.

Moreover, since Table 3 indicates that, when the flux of the presentinvention is used, the content of chlorine, a kind of the halogen, leftin the film after soldering is lower than 0.1%, it is understood thatthe active ion (halogen) in the flux is sufficiently deactivated bymonomer.

In the foregoing examples, an acrylic resin and a styrene-maleic acidresin are used as the thermoplastic resin having a softening point notlower than 80° C. Accordingly, the electrically insulating property andcorrosion resistance can be maintained at high levels in ahigh-temperature atmosphere, and a remaining of the flux after solderingand cracking can be prevented even at a low temperature.

EXAMPLES 15 THROUGH 18

In Example 15, an acrylic resin having an acid value of 100 was used asthe carboxyl group-containing resin in an amount of 11.1% by weight, andbisphenol A diglycidyl ether was used as the epoxy resin in an amount if7.4% by weight. As the activator, 1.2% by weight of adipic acid and 0.3%by weight of propylamine hydrochloride were used, and as the solvent,10% by weight of toluene and 70% by weight of isopropyl alcohol wereused. These components were uniformly mixed and dissolved to obtain aflux of Example 15. The non-tackiness, spreading ratio, insulationresistance, corrosion resistance and solderability of the flux, thecrosslinking degree of the flux film and the chlorine content in thefilm were tested. The non-tackiness, spreading ratio, insulatingproperty and corrosion resistance were determined according to JISZ-3197. At the solderability test, a paper-phenolic substrate (90 mm×135 mm) was soldered by a jet soldering apparatus, and defects werechecked by visual observation. At the test for determining thecrosslinking degree of the film, a flux-coated copper sheet (100 mm ×200mm) was heated by a hot plate, the coated copper plate was extractedwith acetone, the dry weight of the insoluble component was measured andthe gel proportion (%) was calculated. The chlorine content in the filmwas determined according to JIS Z-3197 after extraction of the solublecomponent with methanol.

In Example 16, 7.8% by weight of an acrylic resin having an acid valueof 100 was used as the carboxyl group-containing resin, and 5.2% byweight of bisphenol A diglycidyl ether was used as the epoxy resin.Furthermore, 5.5% by weight of disproportionated rosin was used. Thesame activators and solvents as used in Example 15 were used in the sameamounts as used in Example 15. These components were uniformly mixed anddissolved to obtain a flux of Example 16. The flux was tested in thesame manner as described in Example 15, and the results are shown inTable 4.

In Example 17, 11.1% by weight of an acrylic resin having an acid valueof 60 was used as the carboxyl group-containing resin and 7.4% by weightof bisphenol A diglycidyl ether was used as the epoxy resin. The sameactivators and solvents as used in Example 15 were used in the sameamounts as described in Example 15. These components were uniformlymixed and dissolved to obtain a flux of Example 17. The flux was testedin the same manner as described in Example 15, and the results are shownin Table 4.

In Example 18, 13.0% by weight of an acrylic resin having an acid valueof 60 and an epoxy equivalent of 300 was used as the resin containingcarboxyl and epoxy groups, and 5.5% by weight of disproportionated rosinwas used. The same activators and solvents as used in Example 15 wereused in the same amounts as described in Example 15. These componentswere uniformly mixed and dissolved to obtain a flux of Example 18. Theflux was tested in the same manner as described in Example 15, and theresults are shown in Table 4.

COMPARATIVE EXAMPLE 4

In Comparative Example 4, disproportionated rosin was used in an amountof 18.5% by weight, and the same activators and solvents as used inExample 15 were used in the same amounts as described in Example 15.These components were uniformly mixed and dissolved to obtain a flux ofComparative Example 4. The flux was tested in the same manner asdescribed in Example 15, and the results are shown in Table 4.

                                      TABLE 4                                     __________________________________________________________________________                                                     Comparative                                      Example No.                  Example No.                                      15     16     17     18      4                            __________________________________________________________________________    Compounds                                                                            carboxyl group-                                                                            11.1   7.8    11.1   --      --                           (% of weight)                                                                        containing resin                                                       of Flux                                                                              epoxy resin   7.4   5.2     7.4   --      --                                  resin containing carboxyl                                                                  --     --     --     13.0    --                                  and epoxy groups                                                              disproportionated rosin                                                                    --     5.5    --      5.5    18.5                                adipic acid   1.2   1.2     1.2    1.2     1.2                                propylamine hydrochloride                                                                   0.3   0.3     0.3    0.3     0.3                                toluene      10.0   10.0   10.0   10.0    10.0                                isopropyl alcohol                                                                          70.0   70.0   70.0   70.0    70.0                         kind of carboxyl group-containing resin                                                           acrylic                                                                              acrylic                                                                              acrylic                                                                              --      --                                               resin having                                                                         resin having                                                                         resin having                                                    acid value                                                                           acid value                                                                           acid value                                                      of 100 of 100 of 60                                       kind of epoxy resin bisphenol A                                                                          bisphenol A                                                                          bisphenol A                                                                          --      --                                               diglycidyl                                                                           diglycidyl                                                                           diglycidyl                                                      ether  ether  ether                                       kind of resin containing carboxyl                                                                 --     --     --     acrylic --                           and epoxy groups                         resin having                                                                  acid value of                                                                 60 and equiva-                                                                lent of 300                          Test   non-tackiness                                                                              no cloth                                                                             no cloth                                                                             no cloth                                                                             no cloth                                                                              large cloth                  Results             trace  trace  trace  trace   trace                               spreading ratio (%)                                                                        93     95     93     94      95                                  insulation                                                                          initial stage                                                                        7 × 10.sup.12                                                                  3 × 10.sup.12                                                                  4 × 10.sup.12                                                                  6 × 10.sup.12                                                                   .sup. 6                                                                       × 10.sup.10                   resistance                                                                          50 hrs 6 × 10.sup.12                                                                  1 × 10.sup.12                                                                  2 × 10.sup.12                                                                  5 × 10.sup.12                                                                   4 × 10.sup.8                  (Ω)                                                                           500 hrs                                                                              6 × 10.sup.12                                                                  1 × 10.sup.12                                                                  2 × 10.sup.12                                                                  4 × 10.sup.12                                                                   5 × 10.sup.7           Test   corrosion                                                                           50 hrs no corrosion                                                                         no corrosion                                                                         no corrosion                                                                         no corrosion                                                                          pitting                      Results                                                                              resistance                                                                          500 hrs                                                                              no corrosion                                                                         no corrosion                                                                         no corrosion                                                                         no corrosion                                                                          corrosion                           solderability                                                                              below 1                                                                              below 1                                                                              below 1                                                                              below 1 below 1                             (defect ratio, %)                                                             crosslinking degree of                                                                     42     30     48     38       0                                  firm                                                                          chlorine                                                                            before  0.55   0.55   0.55   0.55    0.55                               content                                                                             soldering                                                               (%) in                                                                              after    0.1>  0.1>    0.1>   0.1>   0.48                               residue                                                                             soldering                                                        __________________________________________________________________________

As apparent from the results shown in Table 4, in each of the foregoingexamples, a soldering flux having an excellent insulating property andcorrosion resistance was obtained.

Furthermore, if the process of the foregoing examples is adopted, sincethe flux residue left after soldering need not be washed away, thewashing step can be omitted, the cost can be reduced, and anenvironmental pollution with flon or the like can be prevented.

The flux of the present invention is valuable as a solder flux for jetsoldering, paste soldering and the like.

INDUSTRIAL APPLICABILITY

Even if the soldering flux of the present invention is exposed in ahigh-temperature atmosphere maintained at a level not lower than 80° C.,such as an atmosphere in an engine room of an automobile, problems suchas a corrosion of the metal surface do not arise, and accordingly, thewashing step can be omitted and the running cost of the soldering stepcan be reduced, and an environmental pollution with a flon detergentdoes not occur. Accordingly, the flux of the present invention isvaluable for use when soldering on an industrial scale, especially forautomobiles.

What is claimed is:
 1. A soldering flux comprising at least (a) athermoplastic synthetic resin having a softening point not lower than80° C. and (b) an activator.
 2. A soldering flux as set forth in claim1, wherein the thermoplastic resin is an acrylic resin and/or astyrene-maleic acid resin obtained by polymerizing or copolymerizing amonomer having a polymerizable unsaturated group.
 3. A soldering flux asset forth in claim 1, which comprises 0.5 to 80% by weight of thethermoplastic resin and 0.1 to 20% by weight of the activator.
 4. Asoldering flux comprising at least (a) at least one compound selectedfrom the group consisting of epoxy group-containing compounds,radical-polymerizable unsaturated double bond-containing compounds andblocked isocyanate group-containing compounds and (b) an activator.
 5. Asoldering flux as set forth in claim 4, which comprises 1 to 70% byweight of at least one compound selected from the group consisting ofepoxy group-containing compounds, radical-polymerizable unsaturateddouble bond-containing compounds and blocked isocyanate compounds, and0.1 to 30% by weight of the activator.
 6. A soldering flux comprising atleast (a) a thermoplastic resin having a softening point not lower than80° C., (b) at least one compound selected from the group consisting ofepoxy group-containing compounds, radical-polymerizable unsaturateddouble bond-containing compounds and blocked isocyanate group-containingcompounds, and (c) an activator.
 7. A soldering flux as set forth inclaim 6, which comprises 0.5 to 80% by weight of the thermoplasticresin, at least 1% by weight of said at least one compound and 0.1 to30% by weight of the activator.
 8. A soldering flux comprising at leastone thermosetting resin selected from the group consisting of athermosetting resin composed of a mixture of a carboxyl group-containingresin and an epoxy resin and a thermosetting resin having a carboxylgroup and an epoxy group, and an activator.
 9. A soldering flux as setforth in claim 8, which comprises 0.5 to 70% by weight of thethermosetting resin and 0.1 to 30% by weight of the activator.